The present invention relates to a spool type directional control valve (spool valve) which decreases a fluid force acting on a spool.
The directional control valve of a spool valve type has a construction in which flow paths are switched over by a spool sliding axially in a slide hole in a valve body.
More specifically, for example, in a four-port directional control valve (four-port switching valve), as shown in FIG. 12, provided in a slide hole 12 formed in a valve body 11 are an annular supply port 14, two annular load ports 15, 16 disposed symmetrically with respect to the supply port, and annular return ports 17, 18 disposed on the opposite outsides of the load ports.
A spool 31 is received in the slide hole, and is held at a neutral position by springs 24a, 24b provided on both sides of the valve body 11. The spool 31 is formed with an annular central concave portion 33 and two land portions 32a, 32b disposed symmetrically on both sides of the central concave portion 33. At the neutral position of the spool 31, the central concave portion 33 opens to the supply port 14, and the land portions 32a, 32b block off the load ports 15, 16 from the supply port 14. The spool 31 is moved axially in the slide hole 12 by solenoids 22a, 22b provided at the left and right of the valve body 11.
When the spool 31 moves, the supply port 14 is bought into communication with either one of the load ports 15, 16, and at the same time, the other of the load ports 15, 16 is allowed to communicate with the adjacent return port 17 or 18. Thus, the spool valve selectively supplies hydraulic fluid from the supply port to one load port, and selectively returns the hydraulic fluid from the other load port to the return port to perform the switching of the hydraulic fluid. Another spool shape may be used to appropriately change the selection of flow paths.
In the above-described directional control valve, as the hydraulic fluid spurts out from the supply port to the load port, a fluid force that resists the spool operating force acts on the spool. Various measures have been proposed for decreasing this fluid force.
Shown in FIG. 13 is a basic construction for this end, in which fluid compensating pockets 42, 43 utilizing a return flow are provided on the return port side at both ends of the spool 41. FIG. 13 shows a state in which a spool 41 moves to the right, fluid flows from the supply port 14 to the load port 16, returns to the load port 15 via a load 44, and flows into the return port 17. When the hydraulic fluid flows from the supply port 14 to the load port 16, pressure unbalance occurs between both wall surfaces of the spool, and resultantly a force FB acting to close the spool 41 is created.
At this time, the flow from the load port 15 to the return port 17 includes a flow 45 going from the load port 15 into the return port 17 at an angle xcex81 and a flow 46 going along an inclined surface 42a of the pocket 42 at an angle xcex82. Taking the spool moving direction as positive, the axial force of the spool 41 generated by these flows is expressed as
Ff1=xe2x88x92Qxcfx81(U1 cos xcex81xe2x88x92U2 cos xcex82)
where, Q is flow rate, U1 is flow velocity of the flow 45, U2 is flow velocity of the flow 46, and xcfx81 is density of hydraulic fluid. In the above-described construction, the relationship of xcex81 greater than xcex82 is adjusted by the angle of the inclined surface 42a so that Ff1 is in the positive direction, that is, in the spool moving direction. Thereby, Ff1 and the fluid force FB counteract each other. Further, by generating a swirl flow 47 in the return port, a force Ff2 in the spool moving direction is provided to reduce the fluid force.
Such a construction is found, for example, in xe2x80x9cHydraulic Pressure Controlxe2x80x9d, Toshio Takenaka and Eizo Urata, 2nd edition, 4th issue, Maruzen Co., Ltd., Feb. 20, 1986, pp. 88-89.
In this construction, when there is no flow returning to the return port 17, 18, the effect of reducing fluid force is not achieved. Also, the angles xcex81 and xcex82 are often determined so that an ideal fluid force reducing effect can be achieved when the rate of flow going from the supply port 14 to the load port 16 and that going from the load port 15 to the return port 17 are equal to each other. Therefore, when the return flow rate is lower than the flow rate on the supply side, for example, in the case where an actuator to be connected to a spool valve is a cylinder, etc. having a difference in pressure receiving area, this effect is little. Contrarily, when the return flow rate is higher than the flow rate on the supply side, this effect is too great, which presents a problem in that, for example, a force in the opposite direction is created.
Thereupon, a construction for solving this problem has been proposed, for example, in JP-A-9-269084. In this construction, as shown in FIG. 14, a wide central land 53 is formed in the center of a spool 51, and concave portions communicating with the load ports 15, 16 are formed on both sides of the central land 53. In addition, fluid compensating pockets 54, 55, which are similar to the pockets 42, 43 shown in FIG. 13, are provided in these concave portions. By the above-described construction, fluid compensation is made between the supply port 14 and the load port 15 or 16 when fluid is supplied from the supply port 14 to one load port 15 or 16 or to the other load port 16 or 15.
The above-described solution has a problem that since the supply port 14 is closed by the wide central land 53, the construction is complex, and the flow path from the supply port 14 to the load port 15 or 16 is restricted, and the pressure loss is high. Also, in what is called a standard type switching valve having an attachment surface of a hydraulic four-port switching valve specified in ISO4401, because of the dimensional limitation, the length between the ports is restricted, so that it is difficult to provide the pocket 54, 55 having a proper shape at two places.
Thereupon, for example, JP-A-11-82767 has proposed another construction. In this construction, as shown in FIG. 15, like the constructions shown in FIGS. 12 and 13, a spool 61 is provided with a central concave portion 63 and two lands 62a, 62b for opening/closing the load ports, which are formed on both sides of the central concave portion, and furthermore, in the central concave portion 63 of the spool 61, a collar-shaped land 64 narrower than the supply port is provided.
However, in this construction as well, the flow path near the supply port 14 is restricted by the collar-shaped land 64, so that there still arises a problem of high pressure loss. Also, this construction has a problem in that the length in the axial direction of the spool must be ensured to provide the above-described pockets 65, 66 on both sides of the collar-shaped land 64. In addition, the construction is complex.
An object of the invention, in view of the above problems, is to provide a spool valve which is suitable for wide use, has a simple construction and a low pressure loss, and is provided with a spool which can provide fluid force reduction between a supply port and a load port, without increasing the axial length, especially the length between ports.
In order to attain the above object, the present inventors provided a pocket in the central concave portion of a spool, and conducted studies on flow angles xcex81 and xcex82 and a swirl flow in the pocket in a state of outflow from the supply port to the load port, which is an opposite flow to the before-mentioned inflow on the return side. As a result, a knowledge was obtained that in the case of outflow from the supply port to the load port, even when xcex81=xcex82, a fluid force decreasing effect enough for practical use is achieved.
Based on this knowledge, the spool valve according to the invention has a fluid force reducer provided in the concave potion of a spool for decreasing a fluid force which acts against the operating force of the spool when the spool valve is actuated. This reducer comprises a spool portion formed so as to let hydraulic fluid smoothly flow along its surface, which is flowing from a supply port to a lord port that is to be supplied with the hydraulic fluid.
This spool valve, since the fluid force reducer is provided only in the concave portion of the spool, can effectively decrease the fluid force acting on the spool during the operation of the valve by a simple construction in which the length in the axial direction is restrained.
The spool valve according to another aspect of the invention comprises a valve body defining an elongate slide hole and a spool slidably received in the slide hole of the valve body. The slide hole has an annular supply port, two annular load ports, and two annular return ports, which are respectively formed in the inner surface of the slide hole. The load ports are disposed symmetrically with the supply port interposed therebetween, and the return ports are respectively disposed on both outer sides of the load ports. The spool has a central concave portion communicating with the supply port of the slide hole and two land portions provided symmetrically with the central concave portion interposed therebetween. The land portions are disposed so as to block off the load ports from the supply port when the spool is at a neutral position. The spool is movable at least between the neutral position and a first position where with the displacement of the land portions, the supply port is allowed to communicate with one of the load ports via the central concave portion and the other load port is allowed to communicate with the adjacent return port. The central concave portion of the spool is formed so that the outside diameter thereof gradually increases from the center in the axial direction of the concave portion toward at least the land portion on the other load port side.
In the above spool valve, at least a half of the central concave portion, which approached the supply port when the valve is actuated, has a shape such that the outside diameter gradually increases from the supply port toward the load port to which hydraulic pressure is to be supplied, that is, an inclined shape. The hydraulic fluid passing through the supply port flows in the central concave portion along this inclined portion, and reaches the load port. Therefore, the hydraulic fluid scarcely stagnates in the central concave portion, and also such a flow exerts an influence on the outflow angle to a restriction or throttling portion between the load port and the corresponding land portion, thereby bringing about a fluid force decreasing effect.
The outside diameter gradually increasing portion in the central concave portion of the spool preferably has a conical shape in which the outside diameter increases lineally. This shape provides simple machining and high accuracy. Alternatively, the outside diameter gradually increasing portion may be in a shape of Japanese hand drum in which the outside diameter increases arcuately. Further, the outside diameter gradually increasing portion need not necessarily have a continuous surface, and may have a stepped columnar shape in which the outside diameter increases discontinuously.
The gradually increasing rate of the outside diameter is preferably set, in terms of one-side angle with respect to the spool center axis, at an angle which is not less than 15 degrees and not exceeding 35 degrees, and the maximum outside diameter is preferably not less than 0.6 and not exceeding 0.8 of the outside diameter of the adjacent land portion. 24 degrees is optimum for the gradually increasing rate of the outside diameter. If the rate is less than 15 degrees or exceeds 35 degrees, the fluid force decreasing effect deteriorates. Optimally, the maximum diameter of the outside diameter gradually increasing portion in the central concave portion is not less than 0.7 of the outside diameter of the adjacent land portion. If it is smaller than 0.6, the flow velocity decreases, and the fluid force decreasing effect is little. If it exceeds 0.8, the flow path is restricted at the inlet of the load port, and the pressure loss increases.
As for the spool valve of two-position type in which a pressure fluid is supplied from the supply port to only one load port, it is sufficient to provide the outside diameter gradually increasing portion only on one side of the central concave portion of spool. On the other hand, in the spool valve of three-position type in which the supply of hydraulic pressure is switched over from the supply port to either of the two load ports, the central concave portion of the spool is preferably formed in a shape symmetrical with respect to the center in the axial direction.
By forming the central concave portion of the spool in the symmetrical shape, the whole length in the axial direction of the supply port can be made one pocket, and thus a dimensional margin can be provided as compared with the above-described conventional spool valve, so that fluid force reduction can be made effectively. Further, since the supply from the supply port to the load ports is accomplished through the same fluid paths at right and left, an equal fluid force decreasing effect can be achieved for both load ports.
In this case, the outside diameter of the central concave portion gradually decreases from both land portions to the center. Therefore, if the length between the land portions is large as compared with the diameter of the land portions so that the diameter at the center is too small, the velocity of fluid flowing along the surface of the central concave portion becomes small, which decreases the fluid force decreasing effect. Further, it is preferable that the spool have a higher strength and the machining amount be smaller. Therefore, it is preferable that a parallel portion, that is, a portion in which the outside diameter is constant, be provided in the vicinity of the center of the central concave portion to prevent the minimum diameter from becoming too small.
The invention can be applied to not only the valve in which hydraulic pressure is supplied to only one load port as described above and the valve of three-position type including the center position but also a two-position directional control valve in which the spool is switched over only between the first position and the second position.
Although the spool valve of the invention has the fluid force reducer provided in the central concave portion of the spool, a fluid force compensating pocket may further be added on the return side of the spool as in the conventional spool valve. The use of the additional fluid force compensating pocket on the return side of the spool can compensate a higher fluid force. On the other hand, in the case where the fluid force compensating pocket is not provided on the return side of the spool, the axial length in the vicinity of the return ports becomes short. Also, in the present invention, even when return fluid is not generated, the fluid force can be decreased.
These and other features and advantages of the invention will be more apparent from the description of embodiments which will be made below with reference to the accompanying drawings.